The present invention relates to a semiconductor device, and more particular, to a semiconductor memory apparatus with a delay locked loop circuit.
A semiconductor memory apparatus embodied in a system which includes a plurality of semiconductor devices is used for storing data. The semiconductor memory apparatus outputs data stored in a cell corresponding to an address outputted from a data requesting unit such as a central processing unit (CPU) or stores data provided from the data requesting unit into the cell.
As an operational speed of a system including a plurality of semiconductor devices has increased and a technology related to a semiconductor integrated circuit has advanced, there has been a demand for increasing data access speed of the semiconductor memory apparatus. In order to access data of semiconductor memory apparatus with a high speed, a synchronous memory device which receives a system clock and can access data every time synchronized with transition of the received system clock has been proposed. Nevertheless, the synchronous memory device cannot meet data access speed required by the system, particularly, the data requesting unit included in the system. Accordingly, a double data rate (DDR) synchronous semiconductor memory apparatus is proposed which can access data every rising edge and falling edge of the system clock.
The DDR synchronous semiconductor memory apparatus should receive or output two data within one cycle of the system clock, since the DDR synchronous semiconductor memory apparatus accesses one data every transition of the system clock. That is, the DDR synchronous semiconductor memory apparatus should output or receive data every time exactly synchronized with the rising edge and falling edge of the system clock. Typically, the output circuit of the DDR synchronous semiconductor memory apparatus receives the system clock through a clock transfer path including a clock buffer, clock transmission lines, and the like inside the DDR synchronous semiconductor memory apparatus and outputs data synchronized with the received system clock.
However, the system clock reaches the output circuit of the DDR synchronous semiconductor memory apparatus with an essential delay occurring while passing through the clock input buffer and the clock signal transmission lines arranged in the DDR synchronous semiconductor memory apparatus. Although the output circuit outputs data in synchronization with the reached system clock, data delayed by the essential delay are inputted to an external device receiving data from the DDR synchronous semiconductor memory apparatus.
To solve above described problem, the DDR synchronous semiconductor memory apparatus includes a circuit, i.e., a delay locked loop circuit for adjusting the essential delay by locking a delay of the transferred system clock. The delay locked loop compensates for a delay value caused by the clock transfer path of the semiconductor memory apparatus. The delay locked loop circuit detects a delay value while an inputted system clock passes through the clock transfer path including the clock input circuit, the clock signal transmission lines, and the like and delays the inputted system clock for a delay time corresponding to the detected delay value to thereby output the delayed system clock into the output circuit. That is, the delay locked loop circuit intentionally delays and locks the inputted system clock by a constant delay value depending on an amount of the detected delay value to output the delay locked clock into the output circuit. Then, the output circuit outputs a data synchronized with the delay locked clock. The data outputted from the output circuit is exactly transferred to the external device, synchronized with the system clock.
Substantially, the delay locked clock is inputted to the output circuit at one previous cycle before the data is outputted; and, synchronized with the delay locked clock, the output circuit outputs the data. Namely, for synchronizing the outputted data with the system clock, the data should be outputted faster than the system clock is delayed on the clock transfer path. Outside the semiconductor memory apparatus, the outputted data are exactly synchronized with rising and falling edges of the system clock, input to the semiconductor memory apparatus. Consequently, the delay locked loop is a circuit for determining how much the data is outputted faster than the timing of data output in order to compensate the delay value of the system clock inside the semiconductor memory apparatus.
FIG. 1 illustrates a timing diagram describing an operation of a delay locked loop for use in a conventional semiconductor memory apparatus.
As shown, the delay locked loop generates a delay locked clock DLL_OUT whose phase leads that of inputted internal clock CLKI. Herein, the internal clock CLKI is generated based on an external clock CLK0 from a clock buffer or an internal clock generator. The semiconductor memory apparatus outputs data D0, D1, and D2, synchronized with the delay locked clock DLL_OUT. If the data D0, D1, and D2 are synchronized with the delay locked clock DLL_OUT, the data can be also synchronized with the external clock CLK0.
Generally, the delay locked loop includes a delay line for delaying an internal clock, a delay line controller for controlling a delay amount of the delay line, a delay model for delaying an output of the delay line by an estimated amount, i.e., how long time the internal clock is delayed by a clock path inside the semiconductor memory apparatus, and a phase comparator for comparing a phase of the internal clock with that of an output of the delay model.
Meanwhile, as a frequency of system clock inputted to the semiconductor memory apparatus is higher, an operation margin for outputting data decreases in the DDR synchronous semiconductor memory apparatus which outputs plural data in synchronization with rising and falling edges of the system clock. Thus, it is important to adjust a duty ratio of the delay locked clock outputted from the delay locked loop in the semiconductor memory apparatus. If the duty ratio of the delay locked clock is set to 50:50, the semiconductor memory apparatus has a stable operation margin for outputting data. Recently, the delay locked loop in the semiconductor memory apparatus includes a circuit for adjusting or controlling a duty ratio of the delay locked clock.
However, it is difficult to output the delay locked clock having an adjusted duty ratio of 50:50 because of a temperature variation and a fluctuation of inputted power voltage level during an operation of the semiconductor memory apparatus and a change of circumstance and condition on operation of a system having the semiconductor memory apparatus. If the duty ratio of the delay locked clock outputted from the delay locked loop is not set or fixed properly, the operation margin for outputting data becomes unstable and, as a result, the semiconductor memory apparatus cannot output all data to an external device at every desirably timing, i.e., the rising and falling edges of the system clock.